Django does this out of the box: https://docs.djangoproject.com/en/1.7/topics/serialization/

But DRF adds lots of extra functionality. Even with DRF it's as simple as adding:

    class UserSerializer(serializers.ModelSerializer):
        class Meta:
            model = User

to get full automatic serialization of a model.

> I don't know if serializing child objects is really a good idea

Depends on the specific case. For internal APIs I tend to create endpoints that give a view what it needs in a single request.